1. Field of the Invention
This invention relates to output buffers and more specifically to metal oxide silicon field effect transistor-driven output buffers. 2. Description of the Prior Art
Digital circuitry has often required output buffers to drive output signals from digital logic to other peripheral devices. There are three basic types of output circuits: (a) push-pull; (b) pull-high; and (c) pull-low. The push-pull buffer amplifier is illustrated in FIG. 1a. This circuit receives an input 1 and converts this input 1 into two signals, gate line 11 into device 9 and line 2 which is inputted into inverter 3 which in turn is inputted to device 6. Devices 9 and 6 are metal oxide silicon field effect transistors which when turned on, allow the connected power supply to drive the output line 8 to the output pad 7 which is outputted to the external peripheral device. Therefore if the input is low, which turns on device 9 via gate line 11, the plus voltage at 10 drives the output pad 7 via line 8. If there was a high input on line 1, then line 2 will have a high input to the inverter 3 which will have a low input on line 4 to the gate of device 6, thus turning on device 6 and driving the output pad 7 via output line 8 from the negative power supply 5. If the input on line 1 is low, there will be a plus voltage on the output pad 7 and if the input line 1 is high, there will be a negative voltage on the output pad 7. In addition to inverter 3 driving the gate of device 6, FIG. 1a illustrates a gated capacitor 13 which supplements the charge on line 4 to the gate of device 6 via line 12. The gated capacitor is a device used to provide the supplmental charge in order to decrease the voltage drop across a P channel MOS field effect transistor 6 when transferring a negative signal. The use of the gated capacitor is referred to as "boot-strapping" and is discussed in greater detail in U.S. patent application Ser. No. 335,028 entitled, "Low Power Circuit for Microcomputer", by Jerald Leach and assigned to Texas Instruments.
The second type of buffer configuration is the pull-high configuration. The schematic for this configuration is illustrated in FIG. 1b. The pull-high configuration merely allows a low signal on line 1 to turn on device 9 via line 11, thus allowing a positive voltage to be placed on the output pad 7 through output line 8. While line 2 still feeds an inverter 3, the inverter 3 is not connected to device 6, thus there is either a positive voltage on the output pad 7 when a low input is received or there is no voltage on the output pad when a high input is received.
The pull-down configuration is illustrated in FIG. 1c. A high inputted on line 1 is input into inverter 3 via line 2 thus causing device 6 to turn on and place the negative voltage 5 through output line 8 onto the output pad 7. Note that the gated capacitor 13 is connected as previously discussed. Device 9 which in the push-pull configuration is used to supply the positive voltage to the output pad is incomplete. The gate is not fabricated. Therefore when an input is low, the output pad 7 receives no voltage; however, when the input pad is high, the negative power supply 5 is output to the pad 7. This type of configuration is used for "active low" type of output drive requirements.
The output buffers discussed above are common output insulated gate field effect transistor (IGFET) buffers and are illustrated in U.S. Pat. No. 3,991,305 entitled, "Electronic Calculator or Digital Processor Chip With Multiple Code Combinations of Display and Keyboard Outputs Scan", by Caudel et al, assigned to Texas Instruments. An advancement in modern microcomputer output buffer architecture is illustrated by U.S. patent application Ser. No. 253,957 (TI8841) entitled, "Multiprogrammalbe Input/Output Circuitry", by Jeffrey Bellay et al, now U.S. Pat. No. 4,435763 assigned to Texas Instruments.
It is customary when a user specifies the output buffer for a IGFET logic circuit, the user will specify one of the three buffer configurations. It is common practice to manufacture output buffers for the push-pull configuration and then simply omit the gates of the specified devices to allow either a pull-high or a pull-low output buffer in fabrication as shown in FIGS. 1b and 1c. While this option satisfies the customer desiring a pull-high or pull-low output buffer configuration, a IGFET driving device is wasted. This is illustrated in FIG. 1d which illustrates the actual IGFET layout of the push-pull amplfier in FIG. 1a. Note that the input 1 is received as before and is connected to device 9 which is a IGFET device including a source 15, a drain 17, a metal gate 18 placed over an oxide region 16. When the input signal 1 is low, the gate is turned on by line 11 thus causing the device 9 to conduct the positive current at node 10 to the output pad 7 via line 8 as previously discussed. Likewise, when the input is high, the input to inverter 3 is high on line 2. A low signal is outputted from inverter 3 on line 4 to device 6. Device 6 comprises the source 22, drain 19 and the gate region 21 is over the oxide region 20. The drain 22 is connected to the negative power supply 5. When line 4 is low (i.e., the input 1 is high) gate 21 turns on. The charge on gate 21 is supplmented by a gated capacitor 13 via line 12. The gated capacitor receives an input from a clock O 42 which is connected to diffusion 24 and is capacitively coupled to a metal region 23 which is connected by line 12 to gate 21. Thus when gate 21 turns on the device 6, the power supply 5 is connected to the output pad 7 via line 8. When the customer desires a pull-high output buffer, the gate oxide 20 is not fabricated. Thus the device 6 which is present on the semiconductor substrate is unused and wasted. Likewise when the customer specifies a pull-low output buffer configuration, device 9 is effectively disabled by not fabricating the gate oxide region 16. The silicon area occupied by device 9 is then wasted.
It is an object of this invention to allow for the alterations of the buffer configurations to utilize both current driving devices.